The present invention relates to radio frequency (RF) dual-reflector antennas. These antennas comprise in general a concave primary reflector of great diameter exhibiting a surface of revolution, and a convex sub-reflector of lesser diameter situated in the vicinity of the focal point of the primary reflector. These antennas operate equally well in transmitter mode or in receiver mode, corresponding to two opposite directions of RF wave propagation. In the following, the description is given either in transmission mode or in reception mode of the antenna, according to whichever one better illustrates the described phenomena. It should be noted that all of the arguments apply just as well to both receiving antennas and transmitting antennas.
The first antennas only had a single reflector, usually parabolic. The end of the radio frequency waveguide is located at the reflector's focal point. The waveguide is inserted into an opening situated on the axis of the reflector, and its end is folded to 180° in order to be opposite the reflector. The maximum half angle of radiation, at the folded end of the waveguide for lighting up the reflector is low, in the region of 70°. The distance between the reflector and the end of the waveguide should be sufficiently extensive to permit the lighting up of the entire surface of the reflector. For these shallow reflector antennas, the F/D ratio is in the region of 0.36. In this ratio, F is the focal length of the reflector (distance between the vertex of the reflector and its focal point) and D is the diameter of the reflector.
In these antennas, the value of the diameter D is determined by the central operating frequency of the antenna. The lower the operating frequency of the antenna (for example 7.1 GHz or 10 GHz) and the more important the diameter of the reflector is for the equivalent antenna gain, the further away the end of the waveguide must be from the reflector to light it up well (transmission mode). The antenna therefore becomes all the more bulky the lower the operating frequency. For these shallow reflector antennas, it is essential to add a dark trace screen in order to minimize the radiation losses by spillover and improve the radio performance.
In order to create a more compact system, one utilizes dual-reflector antennas, in particular those of the Cassegrain type. The dual-reflectors comprise a concave primary reflector, frequently parabolic, as well as a convex sub-reflector having a much lower diameter and placed in the proximity of the focal point on the same axis of revolution as the primary reflector. The primary reflector is bored at its vertex and the waveguide is inserted on the axis of the primary reflector. The end of the waveguide is no longer folded, but rather is opposite the sub-reflector. In transmission mode, the RF waves transmitted by the waveguide are reflected by the sub-reflector to the primary reflector.
It is possible to create sub-reflectors exhibiting a half-angle of illumination of the primary reflector far greater than 70°. For example one can use a half-angle limit of illumination of 105°. In a dual-reflector antenna, the sub-reflector can also be axially quite close to the primary reflector. In practice, the sub-reflector can be situated within the volume defined by the primary reflector, which reduces the space occupied by the antenna.
In these dual-reflector antennas, the utilized F/D ratio is often less than or equal to 0.25. These antennas are called deep reflectors. An F/D ratio in the region of 0.25 corresponds, for an equal value of the central operating frequency D, to a much shorter focal length than is the case where the F/D ratio is close to 0.36. The space occupied by a dual-reflector antenna may well be less than that of a simple reflector antenna thanks to the suppression of the dark trace screen which is no longer essential.
Although the dual-reflector antennas are well adapted to the creation of compact antennas, for example when using the dual-reflectors where the F/D ratio is close to 0.2, one may prefer using the different values of the F/D so as to optimize other characteristics than the occupied space, such as the radiation pattern of the antenna for example.
With a dual-reflector antenna, the sub-reflector should be kept near the primary reflector's focal point. One of the possible ways is to attach the sub-reflector to the end of the waveguide. In this case, the sub-reflector generally consists of dielectric material (usually plastic) more or less cone-shaped and transparent to RF waves. The more or less cone-shaped external surface of the sub-reflector is opposite the primary reflector. The convex internal surface of the sub-reflector is coated with a product enabling the reflection of the RF waves in the direction of the primary reflector when passing through the dielectric material. This coating is usually metallic.
Multiple reflections of the RF waves take place between the end of the waveguide and the primary reflector, involving the sub-reflector. To reduce these reflections, one has proposed introducing local disruptions on the external surface of the sub-reflector opposite the primary reflector. These disruptions have the shape of contours forming rings around the dielectric material. The annular contours are contours of revolution around the axis of the sub-reflector. The profile of these annular contours is made up of crests and projections of different altitudes and depths. These contours can be distributed periodically on the entire external surface of the sub-reflector. However, non-periodic annular contours can be used to modify the reflection characteristics of the sub-reflector, in order to reduce once more the multiple reflections of the RF waves for the two planes of polarization of the electromagnetic wave.
The introduction of annular contours on the external surface of the dielectric material permits the reduction of the multiple reflections of the RF waves which are produced between the waveguide and the primary reflector via the internal metal-plated surface of the sub-reflector. On the other hand, these contours have a lesser effect on two other important properties of the dual-reflector: the antenna gain, expressed in dBi or isotropic decibels, and the losses by spillover, expressed in dB.
In antenna transmission mode, for example, the losses by spillover correspond to the energy reflected by the sub-reflector in the direction of the primary reflector, and whose path ends beyond the external diameter of the primary reflector. These losses lead to a pollution of the environment by the RF waves. These losses by spillover must be limited to the levels defined by the standards.
One customary solution for remedying this is attaching to the periphery of the primary reflector a shroud which has the shape of a cylinder, of a diameter close to that of the primary reflector and of suitable height, coated inwardly with an RF radiation absorbing layer. Besides the congestion which results from it, this known solution exhibits the nowadays awkward drawback of the cost of the shroud material, as well as the cost of the assembly of this shroud on the primary reflector.